In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset. Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio.
The present invention is directed to downshifts, and in particular, to downshifts performed during coast or braking conditions with the engine throttle essentially closed, such shifts being referred to herein as closed-throttle downshifts. Various clutch controls have been proposed for controlling closed-throttle downshifting. For example, the U.S. Pat. No. to Lentz et al. 5,046,174 and U.S. Pat. No. Runde et al. 5,211,079 are directed to a control in which the off-going clutch is released, and the transmission input speed is raised to the synchronous speed of the target speed ratio by a closed-loop control of the on-coming clutch based on the deviation of the on-coming clutch slip from a desired slip profile. In another type of control, disclosed in the U.S. Pat. No. to Downs et al. 4,671,139, the off-going clutch is released after the transmission input speed falls below the engine idle speed, and the closed-throttle engine torque raises the input speed to the synchronous speed of the target speed ratio, whereupon the on-coming clutch is engaged.
While the control disclosed by Downs et al. has the potential of achieving high quality closed-throttle downshifts with no negative output torque disturbance, clutch and pressure control variability make it difficult to consistently achieve the desired timing for off-going clutch dis-engagement and on-coming clutch engagement. This is particularly the case for shifts occurring at very low vehicle speeds, such as the shift to the first or lowest speed ratio. Accordingly, what is needed is a control that produces consistent high quality closed-throttle downshifts in spite of such variability, and that has the ability to adaptively adjust the control parameters for achieving optimum shift quality.